Balko’s last big project for Reason magazine was to help put together a special July issue called “Criminal Injustice: Inside America’s National Disgrace.” It covers many of the issues that he has done such great work on for the last several years. They’ve now put some of those articles online, including one by Balko on wrongful convictions that should be widely read.

He dedicates much of the article to answering a crucial question: How many innocent people have been wrongly convicted and are sitting in prison for crimes they didn’t commit? It’s a difficult question to answer but we can at least bring some evidence and analysis to bear on it.

In a country where there are 15,000 to 20,000 homicides each year, 268 exonerations over two decades may seem like an acceptable margin of error. But reform advocates point out that DNA testing is conclusive only in a small percentage of criminal cases. Testing is helpful only in solving crimes where exchange of DNA is common and significant, mostly rape and murder. (And most murder exonerations have come about because the murder was preceded by a rape that produced testable DNA.) Even within this subset of cases, DNA evidence is not always preserved, nor is it always dispositive to the identity of the perpetrator.

Death penalty cases add urgency to this debate. In a 2007 study published in the Journal of Criminal Law and Criminology, the Seton Hall law professor Michael Risinger looked at cases of exoneration for capital murder-rapes between 1982 and 1989, compared them to the total number of murder-rape cases over that period for which DNA would be a factor, and estimated from that data that 3 percent to 5 percent of the people convicted of capital crimes probably are innocent. If Risinger is right, it’s still unclear how to extrapolate figures for the larger prison population. Some criminologists argue that there is more pressure on prosecutors and jurors to convict someone, anyone, in high-profile murder cases. That would suggest a higher wrongful conviction rate in death penalty cases. But defendants also tend to have better representation in capital cases, and media interest can also mean more scrutiny for police and prosecutors. That could lead to fewer wrongful convictions.

In a study published in the Journal of Criminal Law and Criminology in 2005, a team led by University of Michigan law professor Samuel Gross looked at 328 exonerations of people who had been convicted of rape, murder, and other felonies between 1989 and 2003. They found that while those who have been condemned to die make up just 1 percent of the prison population, they account for 22 percent of the exonerated. But does that mean capital cases are more likely to bring a wrongful conviction? Or does it mean the attention and scrutiny that death penalty cases get after conviction–particularly as an execution date nears–make it more likely that wrongful convictions in capital cases will be discovered?

Many states have special public defender offices that take over death penalty cases after a defendant has exhausted his appeals. These offices tend to be well-staffed, with enough funding to hire their own investigators and forensic specialists. That sometimes stands in stark contrast to the public defender offices that handled the same cases at trial. Perversely, this means that in some jurisdictions, a defendant wrongly convicted of murder may be better off with a death sentence than with life in prison.

Even if we were to drop below the floor set in the Risinger study and assume that 2 percent of the 2008 prison population was innocent, that would still mean about 46,000 people have been convicted and incarcerated for crimes they didn’t commit. But some skeptics say even that figure is way too high…

Whatever the total number of innocent convicts, there is good reason to believe that the 268 cases in which DNA evidence has proven innocence don’t begin to scratch the surface. For one thing, the pace of these exonerations hasn’t slowed down: There were 22 in 2009, making it the second busiest name-clearing year to date. Furthermore, exonerations are expensive in both time and resources. Merely discovering a possible case and requesting testing often isn’t enough. With some commendable exceptions (see “Bad Boys,” page 58), prosecutors tend to fight requests for post-conviction DNA testing. (The U.S. Supreme Court held in 2009 that there is no constitutional right to such tests.) So for now, the pace of genetic exonerations appears to be limited primarily by the amount of money and staff that legal advocacy groups have to uncover these cases and argue them in court, the amount of evidence available for testing, and the willingness of courts to allow the process to happen, not by a lack of cases in need of further investigation.

It’s notable that one of the few places in America where a district attorney has specifically dedicated staff and resources to seeking out bad convictions–Dallas County, Texas–has produced more exonerations than all but a handful of states. That’s partly because Dallas County District Attorney Craig Watkins is more interested in reopening old cases than his counterparts elsewhere, and partly because of a historical quirk: Since the early 1980s the county has been sending biological crime scene evidence to a private crime lab for testing, and that lab has kept the evidence well preserved. Few states require such evidence be preserved once a defendant has exhausted his appeals, and in some jurisdictions the evidence is routinely destroyed at that point.

“I don’t think there was anything unique about the way Dallas was prosecuting crimes,” Watkins told me in 2008. “It’s unfortunate that other places didn’t preserve evidence too. We’re just in a unique position where I can look at a case, test DNA evidence from that period, and say without a doubt that a person is innocent….But that doesn’t mean other places don’t have the same problems Dallas had.”

If the rest of the country has an actual (but undetected) wrongful conviction rate as high as Dallas County’s, the number of innocents in prison for felony crimes could be in the tens of thousands.

This is, indeed, a national disgrace. Unfortunately, there is little political will or interest in doing anything about it.